Internal flow in a WRC car: engine cooling

The main goals of the aero design of a WRC car include to reduce drag and to generate downforce while getting the aero balance of the car, to make it driveable. But there are other goals, not less important, such as feeding adequate flow of air to the engine, efficiently removing exhaust gases or simply cooling the car. Engine is the main source of heat in a WRC car, but there are a lot more devices to be cooled: transmission, brakes, power steering, electric/electronic equipment, power assisted gearshift system or shock absorbers.

An efficient removal of generated heat can make the difference, especially in the hottest events (Mexico, Turkey, Sardegna,…). In this post we will review the internal airflow aerodynamics for engine cooling, while in a next post we will review other devices cooling.

An optimal design of internal flow can bring many advantages to the car, as optimal engine operation requires optimal engine cooling. To reach optimal conditions, a sufficient and well distributed airflow through the heat exchangers of the cooling system is required. Also, an optimal cooling ducts and airflow design means smaller heat exchangers, leading to decreased vehicle weight and reduced drag. Internal flow can become a significant source of drag and lift. It is widely accepted that cooling air can easily generate at least a 10% of the total drag of the car. To minimize it, an adequate design is required, with the minimal pressure loss.

picture by Bruce Thomas – Rallysportmag.com

Citroën C3 WRC, 2018

Airflow enters inside the car through different air intakes in the front of the car. Their size is limited by regulation, specified in the homologation extension form 400/01 WRC, which unfortunately is not made public by FIA. All we can know are the specific regulations for WRC cars set by the Appendix J of Article 255A (2019). There, some conditions on openings configuration are defined, such as

– openings must be fitted wire netting with a mesh of 10 mm maximum,

– electric fans are allowed inside these ducts to enhance air circulation,

– deliberate through-flow of air not permitted except for cooling,

– air ducts (without any modification of the homologated openings) may be added to the front bumper on the following conditions : the air may be channelled only to cool the auxiliaries, and only a single duct per auxiliary is authorised.

All current WRC cars include at least one big, frontal air inlet for engine cooling and feed, together with two small openings on both sides of the central intake, for brake cooling. Location of air inlets is critical to ensure adequate flow of air entering inside the car. For this reason, they are located at the car front, where air can hardly escape from entering inside the car, as the access to the undercar is really restricted, thanks to the front splitter.

Even so, some differences can be found between the solutions used by each of the different manufacturers currently involved in the WRC.

picture by Michelin

S.Ogier/J.Ingrassia, Citroën C3 WRC, Rally Sardegna 2019, 41st

The Citroën C3 WRC shows clearly the three different air inlets at the front of the car: the central, big intake for radiator/intercooler (A), the two side openings for brake cooling (B), and the engine air feed intake on the left side of the Citroën logo, on top of the central intake (C).

picture by Jaanus Ree/Red Bull Content Pool

C.Breen/S.Martin, Citroën C3 WRC, Rally Australia 2018, 7th

Very similar design can be found in the Hyundai i20 Coupe WRC. Main difference is that all intakes have been integrated in the central wire netted opening.

picture by Michelin

T.Neuville/N.Gilsoul, Hyundai i20 Coupé WRC, Rally Sardegna 2019, 6th

picture by Michelin

K.Meeke/S.Marshall, Toyota Yaris WRC, Rally Sardegna 2019, 8th

The Toyota Yaris WRC also follows the same configuration of his rivals, except that the cooling brakes intakes (B) are located on top of the front bumper, on both sides of the engine air feed intake (C), which is located behind the Toyota logo. Another difference is the inclusion of additional air intakes on both sides of frontal intake (D) for additional cooling.

picture by Mfoto.es

O.Tänak/M.Järveoja, Toyota Yaris WRC, Rally Australia 2018, ret.

picture by Michelin

E.Evans/S.Martin, Ford Fiesta WRC, Rally Sardegna 2019, 4th

The Ford Fiesta WRC includes a central air intake (A) split into two, due to the different disposition of the radiator and intercoolers. In the case of the Fiesta, they are placed horizontally, while the rest of the teams include them in parallel in a vertical position.

picture by Bogdan Barabas – eWRC.cz

Ford Fiesta WRC 2017

For an optimal performance of the radiator/intercooler, it is critical to ensure that the velocity distribution reaching the face of the radiator is uniform, for what cooling inlets and ducts need to be designed with this purpose, while keeping the drag as low as possible. An homogeneous airflow distribution is easier to achieve when the radiators are facing the airflow at a completely vertical position, as it is the case of the Fiesta.

All other cars include radiators that are tilted forward at different angles. The reason is to increase the total radiator surface, but at the cost of a poorer flow distribution. The higher the angle, the poorer the distribution, unless airflow is conveniently redirected (which does not seem to be the case). Pictures below show how this angle increases from the Citroën to the Hyundai and even more in the case of the Toyota.

WRC cars engine generate a huge amount of heat during a stage. But in the connections between stages, or during the route to start ceremony, cooling demand decreases a lot. In some cases an excess in cooling brings the engine to work at too low temperatures, which is not recommended. In that cases, some teams (specially Toyota) use the technique known as radiator balnking. It consist in inserting some panels ahead of the front radiator, in oreder to reduce its cooling capacity, to ensure the engine works at an adequate temperature.

picture by Michelin

J.M.Latvala/M.Anttila, Toyota Yaris WRC, Rally Portugal 2019, 7th

When temperature in stage is very low, as it happens usually in Sweden, radiator can be left blanked. But as soon as temperature raises, the panels need to be removed, or they could cause overheating which may quickly lead the engine to failure.

picture by Tapio Lehtonen – rallirinki.net

M.Grönholm/T.Rautiainen, Toyota Yaris WRC, Rally Sweden 2019, 38th

Heat removal capacity of a radiator/intercooler is also a function of the air velocity across it: the higher the velocity the better the heat removal. But, beyond a certain value, the pressure drop becomes too large (resulting in a drag increase). The task of the engineers is to make a design to achieve the highest heat rejection at the lowest possible drag, without reaching the limit. All this have to be taken into account when location and angle of radiator and intercooler are decided.

Hot air generated behind the radiator/inteercooler needs then to be removed. Again, it is important to keep pressure drop low, or we will be increasing drag. To help air to be accelerated, vents are usually located at low pressure areas. The higher the pressure difference, the faster hot air will be removed and the smaller the drag cost. This is the reason why air vents at located at the rear sides of the bonnet, where pressure is low. From this location, hot air is dragged by the main, external airflow towards the windshield, roof and rear wing, contributing to the generation of downforce in the car rear.

Toyota Yaris WRC 2018

Another alternative would be to remove the hot air through the vent of the front fender. And this is what made Toyota, when in 2018 homologated new air vents on top of the front fender. These vents allow to remove engine cooling air on its upper part (1), while brake cooling air on the rear vent of the fender (2). This is also the reason why air vents on the bonnet of the Yaris are more centered, while the other three cars have located their vents on both rear extremes of the bonnet, as shown in pictures above.

Any other alternative which includes removing air through the front wheel space or under the car would have a disturbing effect on the flow under the car, and the result would be a smaller downforce generation, for what they are normally discarded.

Regulations also set conditions for air vents: “Openings on the engine bonnet must be homologated and must be fitted with wire netting with a mesh of maximum 10 mm“.

In a next post, we will review the airflow for brake cooling and the use engineers make of it to improve the aero performance of the WRC cars.